This invention relates to ultrasonic transducers, and, more particularly, to a system for generating and coupling various types of ultrasonic waves with a surface of a workpiece.
An ultrasonic transducer is a device that interconverts electrical and mechanical waves. For example, a 1 megahertz (one million cycles per second) electrical signal fed to such a transducer produces a 1 megahertz mechanical output signal. Conversely, a mechanical signal fed to an ultrasonic transducer produces an electrical output signal. The ultrasonic transducer can therefore be used to produce mechanical waves that are coupled into a workpiece, and/or receive the mechanical waves propagated through the workpiece or along the surface of the workpiece. In either case, the mechanical waves are interconverted with electrical signals that are more readily controlled and/or processed by external electrical circuitry.
In some conventional situations the ultrasonic transducer signal is readily and precisely coupled with the workpiece. For example, if the workpiece is a solid piece of relatively large diameter and the ultrasonic waves of interest are bulk waves that travel through the interior of the solid piece, a transmitting ultrasonic transducer can be placed on one end of the solid piece and a receiving ultrasonic transducer can be placed on the opposite end. The distance of travel between the transmitting and receiving transducers is the distance between the ends of the workpiece.
In other situations coupling between the ultrasonic transducer and the workpiece is not so readily accomplished, nor is the effective propagation distance of the ultrasonic wave so well defined. Where the workpiece is a thin sheet of material, the ultrasonic transducers are placed flat against the surface of the sheet. Since the ultrasonic transducers have a contact area dimension parallel to the direction of propagation, the distance of ultrasonic wave propagation through the sheet is not well defined. Calibration techniques may be used in some cases, but are not practical in others. The same problem arises for complexly shaped workpieces.
An alternative approach is to place the workpiece into a tank of a liquid couplant material. Ultrasonic transducers spaced apart from the workpiece couple with the workpiece through the liquid couplant material. Even with this configuration, propagation distances are also difficult to determine precisely.
There is a need for an approach to introducing ultrasonic waves into thin sheets, plates, and complexly configured workpieces, and receiving them from such workpieces, at precisely defined locations. The technique should be operable with various types of ultrasonic waves. The present invention fulfills this need, and further provides related advantages.